This invention relates to a paper coating composition, more particularly to a cyclic phosphate salt which is added to insolubilize the binder in the paper coating.
Paper coating compositions are generally a fluid suspension of pigment, such as clay with or without titanium dioxide, calcium carbonate, or the like, in an aqueous medium which includes a binder such as starch, modified starch, polyvinyl alcohol, polymers, or protein to adhere the pigment to paper.
The hydrophilic nature of the binder requires the presence of an insolubilizing material which crosslinks the binder upon curing of the coated paper, making it hydrophobic and thus improving the off-set printing characteristics of the surface of the coated paper. The most widely used crosslinking materials are glyoxal resins and formaldehyde-donor agents such as melamine-formaldehyde, urea-melamine-formaldehyde, and partially or wholly methylated derivatives thereof. While these systems are effective, alternative systems are sometimes needed as glyoxal resins are highly reactive and tend to build viscosity and the melamine-formaldehyde resins have an unpleasant odor and release free formaldehyde.
The reaction of various phosphates with binders for use in a variety of applications has been disclosed in the literature. Sodium trimetaphosphate (STMP) has long been used in the detergent industry and the food starch industry. STMP has been used to crosslink granular starch in food use applications such as puddings or pie fillings and has also been used as a wet end additive and a sizepress or water box additive in the paper industry to obtain wet strength. U.S. Pat. No. 2,884,412 discloses preparing a starch phosphate by reacting starch with sodium, potassium or lithium phosphate and using the starch phosphate as a sizing agent in the surface finishing of paper, as a thickening agent in food products, etc. U.S. Pat. No. 3,591,412 discloses coating paper with a pigment and a binder consisting of a depolymerized starch phosphate ester. U.S. Pat. No. 2,699,432 discloses a paper coating composition containing pigment, an alkali metal silicate, latex or starch and tetrasodium pyrophosphate. U.S. Pat. No. 2,801,242 discloses preparing distarch phosphate esters by reacting starch and a metaphosphate salt, with the product useful for pasting papers products, dusting surgeon's gloves, applying adhesives and sizes or in food products. However, these references do not disclose the use of a cyclic phosphate salt as an insolubilizer for binders in paper coating compositions, much less the improved control of coating viscosity and wet rub resistance obtained thereby.